Always crashes

Does anyone knows what the problem is when your computer crashes 90% of the time when you copied large files from drive to drive i.e. c: to d:, and when viewing videos on media player and quicktime etc.

also when downloading files from the internet ex. when im dowloading java offline installation, it crashe even before it hits 2%.

even when viewing hardware properties etc.

I have lots of friends but all of them cant find out the prob. pls help

computer
p4 3 ghz
512 ram
128mb inno3d geforce fx 5200
msi mboard 648f Neo
win xp sp2

Thanks
God speed
John